[UncleScratchy]

There are two merchants in SF - but you probably knew that. The one down the alley has weapons and armor. The Chinese merchant never carries weapons so don't even waste your time with him unless you want other supplies. If the other merchant doesn't have the rifle now just wander out of town and come back after doing an encounter or two. After a few days of traveling try him again. Keep repeating until he finially gets the rifles. Be careful though, those press gangs and cultists can tear you a new one if you run into them. I'm not sure what triggers the change in merchant inventories but this has always worked for me. Both of the SF merchants are easily pickpocketed, as is the upgrade science guy in the cultist camp. Also you can pickpocket some gauss ammo at the NCR from the cops as many of them carry gauss pistols (though you can't steal the pistols). Very rarely if ever will either merchant at the NCR have gauss weapons but they will eventually start getting guass ammo in inventory.